In present-day processes and also in past manufacturing processes, the fibre part of the aforesaid papers comprises/comprised varying quantities of lignin-containing pulp. The remaining fibre quantity normally consists of lignin-free pulp, such as bleached chemical pulp. Lignin-containing pulps, for example high yield pulps, provide several advantages when used as paper ingredients. One advantage is that this type of pulp, even when bleached, is cheaper than chemical pulps in general, and particularly bleached chemical pulps. Another advantage is that several properties of the paper can be improved when the fibre part of the paper includes a given quantity of high yield pulp, as compared with paper which is based solely on chemical pulp. Examples of such properties are opacity (non-transparency), stiffness and bulk.
The big drawback with mixing high yield pulps in paper is that the paper becomes pronouncedly discoloured (yellows) with time. All pulps, and consequently all paper that is based on pulps, will yellow with time. A bleached chemical pulp which contains solely carbohydrates and no lignin at all will also yellow to a certain extent. The extent to which these pulps yellow, however, cannot be compared with the yellowing of pulps that contain lignin. In the case of this latter pulp, yellowing is dominated by the lignin and is accelerated by contact of the fibres with light. This accelerated yellowing of the pulps is caused particularly by the shortwave part of the light, i.e. that part of the light which has a wavelength beneath 425 nm. This drawback of high yield pulps has delayed the use of such pulps in general and/or has at least limited the percentage admixture of such pulps in the fibre part of paper of the aforedescribed type.
Several proposals have been made with regard to limiting the yellowing (improving the brightness stability) of paper which is based completely or partially on lignin-containing pulp.
One method is to reduce the amount of shortwave light, which penetrates the paper. This can be achieved, for instance, by adding to the paper a pigment which has a pronounced ability of spreading shortwave light, as taught in the article "Ny metod mot gulnande papper" by Kenneth Leverback, Kemisk Tidskrift 1990, No. 10, pages 38-39, or by introducing into the paper chemicals which will convert ultraviolet light to heat. This latter type of chemical is normally an organic substance having a phenolic structure.
Another method of limiting the yellowing of lignincontaining pulp and of paper produced therefrom is described in the Swedish Patent Application No. 8700843-9 (EPC Application No. 0 280 332). There is told that bleached lignin-containing pulp is reacted with a reduction agent, for example, and preferably, sodium borydride, in an after treatment process carried out already in the pulp mill. According to the simplest embodiment of the claimed method, a fluroescent chemical, for example an optical whitener, is then introduced to the pulp. This procedure can either be carried out directly on the pulp in the pulp mill or in the paper mill to paper produced from the pulp, for example when surface sizing or surface coating the paper.